1. Field of the Invention
The present invention relates to apparatuses and methods for low-destructively or low-invasively measuring mechanical properties within object such as structures, substances, materials, living tissues (liver, prostate, breast, bone, etc). For instance, measured can be, due to applied stress and/or vibration by arbitrary mechanical sources, the generated displacement vector, strain tensor, strain rate tensor, acceleration vector or velocity vector within the body. Furthermore, from the measured deformation data, the following constants can be measured, i.e., elastic constants such as shear modulus, Poisson's ratio, etc., visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc., delay times or relaxation times relating these elastic constants and visco elastic constants, or density.
In typical applied fields, e.g., in a medical field such as ultra sonic diagnosis, nuclear magnetic resonance diagnosis, light diagnosis, radio therapeutics, the present methods and apparatuses can be applied for monitoring tissue degeneration, i.e., treatment effectiveness. Otherwise, on structures, substances, materials, living tissues, measured static and/or dynamic mechanical properties can be utilized for evaluation, examination, diagnosis, etc.
2. Description of a Related Art
For instance, in the medical field (liver, prostate, breast, bone, etc.), lesions are proposed to be treated by cryotherapy, or by applying radioactive ray, high intensity focus ultrasound, laser, electromagnetic RF wave, microwave, etc. In these cases, the treatment effectiveness is proposed to be monitored. Moreover, chemotherapy effectiveness is also proposed to be monitored (anti-cancer drug, ethanol, etc). For instance, for radiotherapy etc., the treatment effectiveness can be monitored by low-invasively measuring degeneration (including temperature change) of the lesion. Otherwise, due to applied stress to the tissue part of interest including lesions, the generated deformations and deformation changes are measured, from which the pathological states of the tissue are evaluated such as elastic constants etc. Thus, based on the measured distinct pathological states, the part of interest is diagnosed, or treatment effectiveness is observed.
Temperature is known to have high correlations with elastic constants, visco elastic constants, delay times or relaxation times relating elastic constants and visco elastic constants, density, etc. Therefore, by measuring the following constants, the temperature distribution can be measured, i.e., elastic constants such as shear modulus, Poisson's ratio, etc., visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc., delay times or relaxation times relating these elastic constants and visco elastic constants, or density.
In the past, the elastic constants and visco elastic constants have been measured by applying stresses at many points and by measuring the responses such as stresses and strains. That is, a stress meter and/or a strain meter are used, and sensitivity analysis is numerically performed with utilization of the finite difference method or finite element method. Otherwise, in addition to the elastic constants, the visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc. has also been measured by estimating the shear wave propagation velocity generated by applying vibrations.
The disadvantages of the past measurement technique is that the past technique requires many independent deformation fields generated by mechanical sources outside the target body. However, if there exist internal mechanical sources and/or mechanical sources are uncontrollable, the technique becomes unavailable. That is, the past technique requires all information about mechanical sources, such as positions, force directions, force magnitudes, etc. Moreover, the technique requires stress data and strain data at the target body surface, and requires whole body model (using the finite difference method or finite element method). Furthermore, the spatial resolutions of measured elastic constants and visco elastic constants from the shear wave velocity are very low.
In other monitoring techniques for the temperature, evaluated are nuclear magnetic resonance frequencies, electric impedances, ultrasound velocities, etc. However, these techniques require other physical properties of the target tissue to measure the temperature. If the degeneration occurs in the region, the physical properties also change; thus causing severe limitations of the temperature measurement.
On the other hand, a medical ultrasound diagnosis apparatus can low-invasively image a tissue distribution by converting ultrasonic echo signals (echo signals) to images, after transmitting ultrasonic pulses to target tissue and receiving the echo signals by ultrasound transducer. Thus, by ultrasonically measuring the tissue displacements generated due to arbitrary mechanical sources or by measuring the generated tissue strains, tissue elastic constants, etc., the differences of these between lesion and normal tissue can be observed low-invasively. For instance, measured within the body can be, due to applied stress and/or vibration by arbitrary mechanical sources, the generated displacement vector, strain tensor, strain rate tensor, acceleration vector, velocity vector, etc. Furthermore, from the measured deformation data, the following constants can be measured, elastic constants such as shear modulus, Poisson's ratio, etc., visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc., delay times or relaxation times relating these elastic constants and visco elastic constants, or density.
Then, in the past, the tissue displacement has been proposed to be measured to low-invasively diagnose the tissue and lesion by evaluating the echo signal changes of more than one time transmitting signal. The strain distribution is obtained from the measured displacement distribution, based on which the distribution of pathological states of tissue have been proposed to be diagnosed (Japanese Patent Application Publication JP-A-7-55775, JP-A-2001-518342). Specifically, a 3-dimensional (3D), 2D, or 1D region of interest (ROI) is set in the target body, and distributions of three, two, or one displacement components are measured, from which in addition to the strain tensor distribution, the elastic constant distributions, etc. are also numerically obtained.
In addition to the ultrasound transducer, as the displacement (strain) sensor, utilized can be known contact or non-contact sensors such as electromagnetic wave (including light) detector etc. As mechanical sources, compressor and vibrator can be, transducer-mounted apparatuses, not transducer-mounted ones, internal heart motion, respiratory motion, etc. If the ROI is deformed by ultrasound transmitted from sensor, there may not require other mechanical sources except for the sensor. In addition to the stationary elastic constants, the difference of the tissue pathological states includes dynamic changes of elastic constants, temperature due to treatment, etc.
However, as the classical tissue displacement measurement methods assume that tissue deforms or is deformed only in the axial (beam) direction, even when tissue also moves in lateral (scan) direction, the classical method has low axial displacement measurement accuracy. That is, the displacement was determined only by 1D axial processing of the ultrasound echo signals (hereafter, echo signal includes rf echo signal, quadrate detection signal, envelop detection signal, and complex signal).
Recently, the displacement accuracy is improved by us through development of 2D displacement vector measurement method, i.e., the phase gradient estimation method of the 2D echo cross-spectrum based on so-called the 2D cross-correlation processing and the least squares processing. This method can suitably cope with internal, uncontrollable mechanical sources (e.g., heart motion, respiratory motion, blood vessel motion, body motion, etc).
However, strictly speaking, the measurement accuracy of actual 3D tissue displacement becomes low because the method can measure by 2D processing of echo signals two displacement components or by 1D processing one displacement component.
Particularly, as the echo signal has a narrow bandwidth and has no carrier frequency in the lateral direction, the lateral displacement measurement accuracy and spatial resolution are much lower compared with axial ones. Thus, the low lateral measurement accuracy degrades the 3D displacement vector measurement and the 3D strain tensor measurement.
Furthermore, when a large displacement requires to be handled, before estimating the gradient of the cross-spectrum phase, i.e., the phase must be unwrapped. Otherwise, the displacement must be coarsely estimated by cross-correlation method as the multiples of sampling intervals. Thus, the measurement process had become complex one.